Certain austenitic stainless steels and ferritic alloys have properties which make them suitable for use as structural materials in LMFB reactors. Neither of the materials have been found entirely satisfactory. The austenitic stainless steels, which are currently preferred as best able to meet the strength requirements, are known to exhibit a property referred to as "swelling" when subjected to fast neutron irradiation. Also, they lose ductility and have a tendency to brittle fracture on exposure to radiation. Ferritic alloys are considerably better than the austenitic stainless steels with respect to their swelling and ductility properties, and they have the advantages of higher thermal conductivity and lower thermal expansion compared to austenitics. However, the overriding drawback to the conventional ferritic alloys is that they do not have sufficient strength in the temperature range of interest for LMFB reactors which is about 600.degree. to 750.degree. C. The strength required at these elevated temperatures should be preferably at least equivalent to that of 316 stainless steel. In addition, a "candidate alloy" must possess a high degree of room temperature fabricability to facilitate the production of thin walled tubing and other reactor core components. One method of strengthening ferritic steels is by precipitation hardening. It is known that titanium addition of more than 2% titanium to ferritic steel results in the precipitation of a new phase (Fe.sub.2 Ti) which has a strengthening effect. U.S. Pat. No. 3,719,475, for example, concerns a ferritic Fe-Cr-Ti alloy containing about 2% up to 7% titanium. According to the patent, the processing conditions to produce a suitable alloy include a thermo-mechanical treatment to produce precipitation hardening. The tensile strength shown for this material is considerably less than that for 316 stainless steel.
A number of dispersion strengthened ferritic alloys have also been investigated for LMFB reactors. Where titanium has been employed, it has been added in sufficient amounts to form a precipitation hardenable phase. For example, in ISI Special Report 151, pp. 237-241 (1974) and NUCL. TECHNOL. 24 216-224 (1974) investigations are reported on dispersion-strengthened ferritic alloys including titanium-containing materials. The publications report effects of 3.5% and 5% titanium in dispersion-strengthened ferritic alloys. The data show that with respect to high temperature strength, the dispersion-strengthened alloys containing 5% Ti are better than those containing 3.5%. Only one of the alloys containing 5% Ti exceeded the strength of 316 stainless steel at elevated temperature. Despite the good strength exhibited by this material, higher strength would be even more desirable so long as this could be achieved without undue sacrifice of ductility or fabricability.
In accordance with the present invention a ferritic alloy has been found for use in LMFB reactors which not only has the desirable properties attributable to ferritic alloys but also has high temperature mechanical strength and is readily fabricable at ambient temperatures.
In discussion of the invention below, all percentage compositions are given in weight percent.